With increasing environmental awareness, greater priority is being given to cutting fuel consumption and reducing the output of hazardous substances. For the tire manufacturer this means developing tires that are characterized by very low rolling resistance combined with excellent wet skid resistance and good abrasion resistance.
Suggestions for reducing the rolling resistance of a tire and hence fuel consumption have been made in numerous publications and patents. U.S. Pat. Nos. 4,866,131 and 4,894,420 disclose reducing the carbon black content in the compound and using special carbon blacks. However, none of these suggested solutions led to a satisfactory balance between the goal of low rolling resistance and the likewise important tire properties such as wet skid resistance and abrasion resistance.
Only the use of highly active silica fillers in combination with a widespread exchange of the carbon black in the rubber compound appears to offer a route that allows production of a tire with markedly reduced rolling resistance as compared with standard tires combined with retention or even improvement of the two other tire properties mentioned above.
A further improvement in this system with regard to all three properties came with the use of special styrene-butadiene polymers, produced by the solution polymerization method disclosed in EP 0 447 066 A1, in some cases blended with other polymers, in particular polybutadiene, and additional use of novel silica types as disclosed in U.S. Pat. No. 5,227,425 and polymer blends specially tailored for this use as disclosed in EP 0 620 250 A1, with in some cases three to four different starting polymers.
Common to all of these publications and patents is the fact that a large part or the entire content of the carbon black filler that is normally used is replaced by a highly active silica in order to obtain a low rolling resistance while retaining or even improving wet skid resistance and abrasion resistance. In all cases a sulfur-containing organosilane was used as a coupler between the silica and polymer in order to obtain the tire properties that are demanded today.
A typical representative of this class of compound is bis(triethoxysilyl propyl) tetrasulfane (TESPT).
Although the use of bis(triethoxysilyl propyl) tetrasulfane (TESPT) improved important rubber properties such as the dynamic properties that have already been mentioned, this is achieved at the price of a substantially more complicated compounding and processing as compared with carbon black-filled rubber compounds (H.-D. Luginsland “Processing of the Organo Silane Si 69” The International Rubber Chemicals and Compounding Conference, 22–23 Nov. 1999, Antwerp, Belgium).
Thus, silica-filled tire tread compounds containing 3,3-bis(triethoxysilyl propyl) tetrasulfide corresponding to the prior art are at extreme risk of scorching and must under no circumstances exceed the temperature limit of 160° C. when being compounded in an internal mixer.
As a result of this, silica-filled tire tread compounds are always mixed, cooled and stored repeatedly, occasionally up to five times, before the unvulcanised mixes can be accelerated and processed further, whereas in the case of carbon black-filled tire tread compounds the compounds can be processed further after being mixed only twice at elevated mixing temperatures. Silica-filled tire compounds thus give rise to an enormous drop in productivity in the tire manufacturer's entire production process.
It is therefore desirable to produce a compound which enables the improved properties of the silica-filled compounds to be achieved yet nevertheless permits mixing temperatures above 160° C. during processing and thus makes it possible to have fewer mixing stages than previously, as a result of which the productivity of the tire manufacturing process is increased, i.e. the overall production costs of the tire manufacturing process with silica compounds are reduced.
It is also desirable to reduce ethanol emissions, which originate from the low-molecular sulfur-containing organosilanes, during production of the silica compounds.
These objects are achieved by silica-containing rubber compounds according to the present invention and the process for their production.